THE REMEASUREMENT OF THE LOUGH FOYLE BASE LINE

Abstract

IN 1952 when the new primary triangulation of Northern Ireland was observed and connected to the new triangulations of Great Britain and the Republic of Ireland it was not possible, nor considered necessary, to remeasure the Lough Foyle Base line. The line of sight between the terminal stations was blocked. In addition, the figure originally used for the extension of the base line on to the nearest sides of the Principal Triangulation was badly shaped and some of its stations were no longer in existence. It was, however, desirable that the length of the base as measured by Major-General Colby with his compensation apparatus during the years 1827–28, as well as the value accepted by Col. A. R. Clarke, F.R.S. for the calculation of the Principal Triangulation, should be compared with a remeasurement by modern distance measuring equipment. Such equipment, which depends upon the accurate measurement of the transit time of electromagnetic waves, has been developed since the end of the second world war. The most notable instruments in this category are the geodimeter, which employs visible light, and the tellurometer, which makes use of radio waves. In each case the time required by the waves to traverse the double distance between the stations is very accurately ascertained and the distance is then calculated using a calculated value for the velocity of propagation of the waves.